How to insert invisible text into a PDF?

Question

UPDATE: Please see https://softwarerecs.stackexchange.com/questions/71464/java-library-to-insert-invisible-text-into-a-pdf instead.

I want to insert invisible text i

Answer 1

You do not have to render the text invisible. Just render them in the appropriate place but overlay the scanned image on the text. Or, you could render the text over the image and set alpha value of the color of the the stroke and brush to zero.